1. Field of the Technology
The present invention relates to Wavelength Division Multiplexing (WDM) techniques and, more particularly, to an inter-card channel protection method in a WDM system.
2. Background of the Invention
Along with the fast development of WDM techniques and the doubled increment of the transmission rate of WDM system, it is necessary to pay attention to the security and reliability of transmission channels in the network. There are, however, drawbacks in the existing methods of inner-card channel protection as well as methods of inter-card channel protection supported by system control unit.
FIG. 1 shows the solution of one technique in the prior art. FIG. 1a and FIG. 1b are the schematics illustrating the transmitting end and receiving end of inner-card channel protection, respectively. Cards supporting inner-card channel protection include those of Optical conversion unit (OCU) type and Optical transponder unit (OTU) type. At the transmitting end, signal from a card is divided by a coupler into two identical paths, which can be configured such that one is the primary path while the other is the standby path. The signal of the two paths is transmitted via different routes to the receiving end. At the receiving end, both paths of signal enter the card and one of the two is selected. Under normal circumstances, signal of the primary path is selected while the card can make switching depending on the signal quality of the two paths. Refer to G.841 of ITU-T for the external commands and local switching conditions of signal quality.
Drawbacks of this solution are: 1. It is impossible to implement device-level protection, protection can not be provided when there is a failure with the card or with an essential element thereon 2. The unit price of such a card is relatively high, for the card has to be of the type of double outputs/alternative inputs, it has to receive two paths of signal, detect the signal quality of both paths as well as implement within the card the selection of one path from the two. Although the unit price is lower than that of two cards of single output/single input, it is much higher than the price of one card of simplex. In comparison, inter-card channel protection requires two cards of simplex at both the transmitting end and the receiving end while inner-card protection requires one card of dual transmission with selective receiving at both the transmitting end and receiving end. Although inner-card protection costs less, it has little advantage. For the two cards used in inter-card protection may be configured in the non-protection mode and used for transmitting another client signal, respectively, thereby transmitting double amount of data while the card used in inner-card protection can not be used for transmission of other data even if it is configured in the non-protection mode.
FIG. 2a and FIG. 2b show another solution in the prior art. Cards supporting inter-card channel protection supported by system control unit in accordance with this solution include OCU cards and OTU cards. At the transmitting end, signals from the client side are divided by a coupler into two paths and are provided for two cards, respectively. The two paths of signal transmitted from the two cards are sent to the receiving end through different routes. At the receiving end, the two paths of signal enter two cards, respectively, while the system control unit controls the lasers at the client side of the two cards in accordance with the signal qualities submitted by the two cards such that the laser being selected switches on and the laser not selected switches off. Thus, the selected path of signal from the client side of the two cards is transmitted down to the client side. Under normal circumstances, the signal from the primary card is selected. Refer to G.841 of ITU-T for handling of the external commands and local switching conditions of signal quality. This solution, however, has the following drawbacks: 1. Even with proper configuration, the system control unit has to be involved to implement the protection switching. If the system control unit fails, protection will be impossible 2. Since it is needed for the cards to submit to the system control unit the switching conditions and for the system control unit to issue to the card the switching command, the speed is low.